Pulmonary aspergillosis was discovered in apparently healthy young rabbits. Thirty-five rabbits of various ages raised in the same rabbitry were examined by histological, ultrastructural and microbiological methods. The pulmonary lesions were found only in rabbits younger than five weeks old. These lesions contained characteristically shaped hyphae surrounded by eosinophilic radiating projections--"asteroid bodies." The significance of the disease condition and the structure of the "asteroid bodies" is discussed.